Preparation And Properties Of High-strength Editable Shape Memory Hydrogels

As a kind of intelligent material that can respond to physical or chemical stimuli such as light,temperature,p H,humidity,electric field,magnetic field and so on,intelligent responsive hydrogel has great potential in the development of specific applications such as intelligent robots,artificial muscles,structural components,sensors and biomimetic intelligent devices.Among them,stimulus-responsive shape memory hydrogel can be driven by reversible shape change,which has attracted the attention of many researchers.People have designed and fabricated many drivable devices,such as double-layer hydrogel actuator,intelligent robot soft grip,gradient response hydrogel and so on.Because there is a large amount of water in the hydrogel,it is difficult to produce effective energy dissipation under the concentrated action of external forces,which leads to the damage of the hydrogel.Although a series of enhanced hydrogels including double network hydrogels,nanocomposite hydrogels and topological hydrogels have been developed for decades.However,it is still a great challenge to prepare supramolecular hydrogels with high strength,high toughness and stimulus response.In this paper,a strategy with simple operation and universal applicability is reported to prepare responsive high-strength editable shape memory hydrogels.The hydrogel shows uniform and transparent properties in optical properties,ultra-high strength and toughness in mechanical properties,and the mechanical properties can be adjusted in a wide range.Fourier transform infrared spectroscopy（FTIR）was used to analyze the interaction of internal groups in the hydrogel during the variable temperature process,and the reason why MC/PMAA hydrogel still had high strength in the case of complete physical crosslinking was explained from the molecular level.At the same time,the changes of group interaction in the complex and the important influence of water molecules in the system were monitored during the synthesis of hydrogel.In addition,adding photothermal dyes into the MC/PMAA hydrogel gives the hydrogel photothermal response function,which can realize long-distance photocontrol drive.The main research contents are as follows:（1）Preparation of MC/PMAA hydrogels:The strong hydrogen bond interaction between methacrylic acid and methylcellulose was used to form complex precipitates in aqueous solution.The precipitates were collected by high-speed centrifugation and could be shaped arbitrarily.High-strength MC/PMAA hydrogels were prepared by one-step photopolymerization.The transmittance of the hydrogel can reach 87%,and the water content can be adjusted between 30 wt%and 50 wt%.It can be stored stably under water without excessive swelling.（2）Characterization of mechanical properties of hydrogel:The effects of molecular weight of methylcellulose and different content of methacrylic acid on the mechanical properties of hydrogel were investigated.The mechanical strength of hydrogel increased with the increase of molecular weight of MC and decreased with the increase of the content of methacrylic acid.The hydrogel is glassy at room temperature,and there is an obvious yield point in the stretching process.In the temperature range of 20 ^oC to 70 ^oC,the tensile strain,fracture stress and Young’s modulus of the hydrogel varied in the range of 95%-287%,20 MPa～6.5 MPa and 250 MPa～1.5 MPa,respectively.（3）Characterization of the interaction within the hydrogel:It is proved by infrared spectroscopy that there is a strong hydrogen bond interaction between MC and PMAA.The higher the temperature is,the weaker the hydrogen bond interaction is;the higher the water content is,the weaker the hydrogen bond interaction is.（4）Applications of hydrogels:Hydrogels have the shape memory function of multiple responses,and the deformation of hydrogels can be driven remotely by photothermal effect.The non-equilibrium hydrogels are prepared by changing the stress and water distribution of local sites of hydrogels.Double opposite morphological changes were achieved in a single environment.The MC/PMAA hydrogel prepared in this paper has high strength,high toughness and multiple stimulus response,and the preparation process is simple and universal,which provides a new idea for the preparation of a new high-strength hydrogel actuator.